Bosch Furnace: Won't Start or Turn On
Commonly a faulty igniter, a dirty flame sensor, or a blower or control fault. Add a photo of your Bosch Furnace in the box below: the Triage Profile says what it is, how urgent it is, and who to call.
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In the next five minutes
Safe first steps, before the call. Nothing here needs a tool.
If you smell gas, a carbon monoxide alarm sounds, or anyone feels dizzy or sick: don’t touch any switch or plug, leave and get everyone out, then call 911 or the gas company from outside.
No gas smell? Switch the furnace off at the thermostat and at its breaker.
Don’t use it again until it’s checked, and keep people away from it.
Everything else about this problem
Is it an emergency?
Is a Bosch furnace that won't start an emergency?
Usually not, but it can be. Because this can be an emergency, leave it off and get it checked; don't reset it or turn it back on. Stop and call a pro if it keeps tripping the breaker, smells like burning, or you smell gas.
Symptoms & causes
Your Bosch furnace might show no signs of life, meaning the fan isn't blowing, there's no ignition click, and the thermostat display could be blank or showing an error code. This often points to a lack of power, a tripped breaker, a faulty thermostat, or a safety lockout due to an underlying issue like a clogged filter restricting airflow. For older Bosch models, a pilot light that has gone out is a common culprit, while newer high-efficiency units typically rely on an electronic ignition system that may have failed.
Tools & parts
You'll need a basic multimeter to test for voltage at various components and check continuity. A standard screwdriver set will be essential for accessing panels and tightening terminals, for a decent set. Depending on the issue, you might need a new Bosch-compatible furnace filter or a replacement hot surface ignitor if yours is cracked or not glowing.
Safety first
Always begin by shutting off the electrical power to the furnace at the circuit breaker to prevent electrical shock hazards. For gas furnaces, locate and turn off the main gas supply valve to the unit to eliminate any risk of a gas leak during inspection or repair. Be cautious of hot surfaces inside the furnace, especially around the combustion chamber and exhaust vent, to avoid burns, and never bypass safety switches.
When to call a pro
If your Bosch furnace's main control board is unresponsive or displaying complex error codes after basic troubleshooting, it likely requires a professional testing and replacement. A faulty gas valve that isn't opening to supply fuel to the burners, even when the ignitor is working, is a critical component best handled by a certified HVAC technician due to gas safety regulations. Any suspected issues with the heat exchanger, such as cracks or corrosion, demand immediate professional attention as this can lead to dangerous carbon monoxide leaks.
Questions people ask
Why is my Bosch furnace blowing cold air?
This often indicates a problem with the heating element or gas supply, preventing the air from being warmed. Check if the pilot light is out or if the ignitor is failing to glow.
What does a flashing error code on my Bosch furnace mean?
Flashing error codes are warning indicators from the control board, signaling specific issues like pressure switch failures or flame sensor problems. Consult your Bosch furnace's owner's manual for the exact meaning of the code.
How often should I change my Bosch furnace filter?
For optimal performance and air quality, you should typically change your Bosch furnace filter every 1-3 months, especially if you have pets or allergies. A clogged filter can restrict airflow and cause the furnace to overheat and shut down.
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What it is, how urgent, who to call
Add a photo of your Bosch Furnace in the box at the top. The Triage Profile says what it is, how urgent it is, and who to call.